The present invention relates to an electron beam irradiating apparatus used for processing exhaust gas and the like discharged from a thermal power plant, for example, or an electron beam irradiating apparatus for large current irradiation used to refine the quality of substances such as cross-linking of resins. The present invention particularly applies to a method and apparatus for irradiating an electron beam in which the electron beam is moved in a scanning motion while being emitted into the atmosphere through a window foil for ejecting electrons.
It is currently thought that SOx, NOx, and other components found in flue gas that is discharged from thermal power plants and the like is the cause of such global problems as global warming and acid rain that have been linked to air pollution. Methods of desulfurization and denitration remove these toxic components SOx, NOx, and the like through the irradiation of an electron beam on the flue gas are well known in the art.
FIG. 1 shows an example of an electron beam irradiating apparatus used for the above application. This device used for processing flue gas mainly comprises a power source 10 for generating a DC high voltage; an electron beam irradiating apparatus 11 for irradiating an electron beam on the flue gas; and a channel 19 through which the flue gas is transported. The channel 19 is disposed along a window foil 15 that serves as an outlet for the electron beam irradiated from the electron beam irradiating apparatus 11. The window foil 15 is formed of a thin plate constructed of titanium or the like. An electron beam emitted externally through the window foil 15 irradiates such molecules as oxygen (O2) and water vapor (H2O) in the flue gas. These molecules become such highly oxidative radicals as OH, O, and HO2. These radicals oxidize the noxious components of SOx, NOx, and the like and generate the intermediate products of sulfuric acid and nitric acid. These intermediate products react with ammonia gas that has been introduced in advance to produce ammonium sulfate and ammonium nitrate, which can be recovered and used in fertilizer. Accordingly, this type of exhaust gas processing system can remove harmful components, such as SOx and NOx from the flue gas and can recover such useful materials as ammonium sulfate and ammonium nitrate as by-products.
The electron beam irradiating apparatus 11 comprises a thermionic generator 12 such as a thermionic filament; an accelerating tube for accelerating the electrons emitted from the thermionic generator 12; a deflecting coil 16 (electromagnet) for deflecting the electron beam in the widthwise direction by applying a magnetic field using a square wave current; and a scanning coil 17 (electromagnet) for moving the controlled electron beam in a lengthwise scanning direction by applying a magnetic field to the electron beam. Of these, the electron beam generator, accelerating electrode, and deflecting/scanning magnetic poles are accommodated in vacuum vessels 18a and 18b and maintained in a high vacuum atmosphere of approximately 10xe2x88x926 Pa. By supplying an electric current to the deflecting coil 16 and scanning coil 17 and forming a magnetic field using the electromagnets, the high-energy electron beam is injected in a prescribed range through the window foil 15 onto a prescribed area of the channel 19, while deflecting the beam and moving the same in a scanning direction.
As described above, this type of electron beam irradiating apparatus must eject an electron beam highly accelerated in a vacuum environment into the atmosphere. Generally, in order to achieve a high electron transmission efficiency when ejecting an electron beam, a window foil formed of a pure titanium membrane or a titanium alloy membrane having a thickness of several tens of micrometers, for example 40 xcexcm, is used. This window foil is mounted on the end of the vacuum vessel 18a via a mounting flange. The window foil is large, for example 3xc3x970.6 meters. A pressure of approximately 1,000 hPa, which is atmospheric pressure, is applied to the outer surface of the window foil having an inner vacuum pressure in the vacuum vessel of 10xe2x88x926 Pa.
Next, deflection and scanning of the electron beam will be described.
A triangular wave generator 22 supplies a triangular wave current as shown in FIG. 2A to the scanning coil 17 in order to move the electron beam to scan in the Y-direction shown in FIG. 3. A square wave generator 21 supplies a square wave current as shown in FIG. 2B that is synchronized to the triangular wave to the deflecting coil 16 in order to move the electron beam to scan in the X-direction orthogonal to the Y-direction shown in FIG. 3. As both coils 16 and 17 become excited by the currents, the electron beam is accelerated by an accelerating tube 13 and enters the deflection/scanning section to scan along a rectangular path as shown in FIG. 3. The electron beam passes through the window foil 15 and irradiates the target matter.
Here, the path Y1 shown in FIG. 3 is formed when the square wave current between times T1 and T2 in FIGS. 2A and 2B is fixed at +Q and while the current from the triangular wave generator changes from +P to xe2x88x92P. The path X1 is formed when the triangular wave current peaks at xe2x88x92P (time T2) and the square wave current changes instantaneously from +Q to xe2x88x92Q. Similarly, the path Y2 is formed when the triangular wave current changes from xe2x88x92P to +P between times T2 and T3, while the path X2 is formed at time T3, when the square wave current changes instantaneously from xe2x88x92Q to +Q.
FIG. 4 shows the magnetic hysteresis characteristics of the scanning coil 17. When the scanning coil 17 moves the electron beam in the scanning direction, the relationship between the current I and the magnetic flux density B of the scanning coil 17 has hysteresis characteristics at the reversing points in both Y-directions, or in terms of the scanning coil current, at the point of transition when the beam point on the triangular wave current begins to drop or rise. At these points, the flux density B cannot follow the current I, thereby slowing down the scanning rate of the electron beam. Hence, whenever the peak values (+P and xe2x88x92P) of the triangular wave current I enter the saturation region of the flux density B, the flux density B does not change even when the size of the current I changes, thereby causing the scanning rate of the electron beam to change. Accordingly, the amount of electron beam irradiation becomes uneven.
Referring back to the hysteresis characteristics of I and B in FIG. 4, the flux density B does not drop or rise in proportion to rises and falls in the current I, but rather remains relatively uniform for a short time. As a result, the electron beam stagnates during this period. Therefore, the dose at the starting points of each Y scan indicated with hatching in FIG. 3 is increased, causing a non-uniform distribution.
FIG. 5A is a graph plotting the distribution of electron dose along the Y-direction during this time. The graph shows the combined state of Y1 and Y2. As can be seen, there is an unbalanced amount of stress added to the window foil in the irradiating window. This stress causes the temperature at specific areas of the foil surface to rise abnormally, thereby further decreasing the life of the window foil. Further, a uniform electron beam is not applied to the targeted matter beneath the window foil.
Therefore, a method has been proposed for achieving uniformity in the electron irradiation dose that considers the hysteresis delay in the flux density during the drop of the triangular wave. This method performs irradiation with a delta function step (superimposing a kick pulse) near the peak of the triangular wave.
However, simply using a triangular wave with a superimposed kick pulse to even the electron dose does not cancel the non-uniformity of the electron beam dose near the starting points at both ends in the Y-direction. In actual measurements of the electron dose distribution for the electron beam scanning in the Y1 and Y2 directions, a slanted distribution is found, as shown in FIGS. 5B and 5C.
Another conventional method of deflecting and scanning an electron beam will be described with reference to FIGS. 6A and 6B. The triangular wave generator 22 supplies a triangular wave current, such as that shown in FIG. 6A to the scanning coil 17, causing the electron beam to scan in the lengthwise direction (Y-direction) shown in FIG. 7. The square wave generator 21, on the other hand, supplies a trapezoidal wave current, such as that shown in FIG. 6B, to the deflecting coil 16, causing the electron beam to scan in the widthwise direction (X-direction). The triangular wave current shown in FIG. 6A and the trapezoidal wave current shown in FIG. 6B are synchronized such that the peaks of the triangular waves coincide with the midpoints A and Axe2x80x2 in the rise of the trapezoidal wave. Accordingly, the deflecting coil 16 and scanning coil 17 cause the electron beam to scan along an elongated hexagonal path, such as that shown in FIG. 7.
In this case, the electron beam is accelerated in the vacuum vessel and deflected to scan through the window foil and irradiate through the irradiation window onto the target matter in the air. However, energy is lost when the accelerated electron beam passes through the window foil, thereby heating the foil. If the beam is concentrated on one part of the window foil, the heat concentrated at that part could cause the foil to tear. Therefore, it is desirable to maintain a uniform heat density when conducting deflection and scanning of the electron beam. However, reversing points A and Axe2x80x2 in the elongated hexagonal scanning path shown in FIG. 7 correspond to the end of scanning in the Y-direction at the midpoints of the rise of the trapezoidal wave current shown in FIG. 6B. As a result, the electron beam moves in the X-direction at points A and Axe2x80x2, indicated by the hatching in FIG. 7, but turn back in the Y scanning direction. Hence, the movement of the electron beam stagnates in these areas, allowing heat to become concentrated on the window foil and making it possible for the foil to tear.
In view of the foregoing, it is an object of the present invention to provide a method and apparatus for electron beam irradiation capable of performing a uniform scan and avoiding the problems of hysteresis in the scanning coil when scanning the electron beam reciprocally in the lengthwise direction.
It is another object of the present invention to provide a method and apparatus for electron beam irradiation that is capable of avoiding heat concentration caused by the electron beam on the irradiation window.
These objects and others will be attained by an apparatus for irradiating an electron beam comprising a scanning coil; a triangular wave generator for providing a triangular wave current to the scanning coil to move the electron beam in a first scanning direction; a deflecting coil; a square wave generator for providing a square wave current to the deflecting coil to move the electron beam in a second scanning direction orthogonal to the first scanning direction; and a control unit for modulating the triangular wave current provided from the triangular wave generator for canceling the effects of hysteresis in the scanning coil.
Here, the control unit should modulate the triangular wave current to form steep slopes on the rise and fall of the waveform. Further, the waveform of the triangular wave current has a plurality of displacement points on both the rise and fall of the waveform to divide the rise and fall into a plurality of connected linear segments.
According to another aspect of the present invention, a method of irradiating an electron beam comprises the steps of generating a triangular wave current using a triangular wave generator; supplying the triangular wave current to a scanning coil to move the electron beam in a first scanning direction; generating a square wave current using a square wave generator; supplying the square wave current to a deflecting coil to move the electron beam in a second scanning direction orthogonal to the first scanning direction; and modulating the triangular wave current provided from the triangular wave generator using a control unit to cancel the effects of hysteresis in the scanning coil.
Here, the triangular wave current should be modulated to form steep slopes on the rise and fall of the waveform.
The present invention compensates for the relationship between the electric current and the flux density hysteresis in order to achieve a uniform irradiation dose for the triangular wave current used to scan the electron beam in the lengthwise direction. Because of the hysteresis characteristics, the flux density has almost no change in relation to changes in the current during the rise and fall points of the triangular wave current. By forming a steeper change in the electric current at these points, it is possible to avoid the effects of hysteresis and achieve an approximately linear change in flux density. By so doing, it is possible to maintain a substantially fixed scanning rate for the electron beam. The method of the present invention solves the problem in conventional apparatus in which the electron beam stagnates (the scanning rates slows) due to the hysteresis in the scanning coil. Therefore, it is possible to achieve a uniform dose distribution to prevent an unbalance in the dose applied to the window foil.
According to another aspect of the present invention, a method of irradiating an electron beam comprises the steps of generating a triangular wave current using a triangular wave generator; supplying the triangular wave current to a scanning coil to move the electron beam in a first scanning direction; generating a square wave current using a square wave generator; supplying the square wave current to a deflecting coil to move the electron beam in a second scanning direction orthogonal to the first scanning direction; and synchronizing the rise of the square wave current to be shifted a prescribed interval in relation to the peak values of the triangular wave current in order to distribute the reversing points on the electron beam path along the second scanning direction.
Here, the timing of the rise in the square wave current should be shifted each cycle to repeatedly alternate the position on the square wave in relation to a reference rising position in the order of a reference position, a delayed position, an advanced position, the reference position, the delayed position and so on. Further, the reversing point in the electron beam path is moved in order within about half the scanning width formed by the square wave current.
According to another aspect of the present invention, an apparatus for irradiating an electron beam comprises a scanning coil; a triangular wave generator for providing a triangular wave current to the scanning coil to move the electron beam in a first scanning direction; a deflecting coil; a square wave generator for providing a square wave current to the deflecting coil to move the electron beam in a second scanning direction orthogonal to the first scanning direction; and a controller for synchronizing the rise of the square wave current to be shifted a prescribed time interval in relation to the peak values of the triangular wave current to distribute reversing points on the electron beam path in a prescribed order along the second scanning direction.
With this construction, it is possible to spread the reversing positions at which points the electron beam is concentrated, thereby avoiding heat concentration on the window foil. In this way, the life of the window foil is lengthened, and the load placed on the device for cooling the window foil is reduced. As a result, this device can be made more compact. It is also possible to irradiate a uniform electron beam onto the target matter beneath the window foil to generate a homogeneous reaction with the target matter.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.